1. Field of the Invention
The present invention relates generally to hydrophones and, more particularly, to a hardware circuit that provides synchronous demultiplexing of a data stream comprising frames of digital words.
2. Description of the Prior Art
Data acquisition cards can be utilized to demultiplex data streams which comprise frames of digital words wherein the frame includes one or more header words, which may be sixteen bit words that are repeated for each frame. However, the data acquisition cards required to demultiplex the data produced by an eight channel hydrophone array tend to be expensive especially in light of additionally required software development. As well, the large software overhead results in significant processing delays and may also occasionally result in errors, lost data, or other problematic operation.
Another approach to demultiplex such data streams may utilize a microprocessor to detect the header words that act to sync each frame of data, and then apply a clock or operate signal to the eight corresponding digital to analog converters. This approach requires non-trivial processing time to respond to the header words.
The following U.S. Patents describe various prior art systems that may be related to the above demultiplexing problems:
U.S. Pat. No. 4,879,694, issued Nov. 7, 1989, to Grado, discloses a demultiplexer circuit responsive to a composite input signal from a sonobouy. The circuit has a filter and comparator means responsive to the composite signal for providing a filtered frequency pilot reference signal. The filtered frequency pilot reference signal has a frequency equal to one half the frequency of a reference carrier signal in the composite signal. The circuit has a sine demodulator means responsive to the composite signal and to an east/west demodulator signal for providing an east referenced output signal. A cosine demodulator means is responsive to the composite signal and to a north/south demodulator signal for providing a demodulated phase pilot signal and the north referenced output signal. A north/south filter means is responsive to the output signals from the cosine demodulator means for filtering the demodulated phase pilot signal to provide the north referenced output signal. A servo means is responsive to the filtered frequency pilot reference signal and to the demodulated phase pilot signal for generating and controlling the phase of the north/south demodulator signal and the east/west demodulator signal using a single phase locked loop. The servo means is characterized to phase shift the north/south and east/west demodulator signals to adjust the outputs of the sine and cosine demodulator means to remove all error bias measured with respect to ground from the demodulated phase pilot signal.
U.S. Pat. No. 5,291,459, issued Mar. 1, 1994, to Andersen, discloses a hydrophone analog signal data acquisition, A/D conversion and data transmission system that includes a first-stage signal processing subsystem which provides digital representations of the hydrophone analog signal, which in turn are signal processed for transmission in the form of data packets by a second stage signal processing subsystem. A subsystem includes a plurality of Data Multiplexer/FIFO units, including corresponding selectively acting data unit accumulators, each accumulator having a plurality of inputs coupled to output channels of the first-stage signal processing subsystem for receiving digital representations of hydrophone analog signals. Each data unit accumulator includes a first buffer for storing information that includes a digital representation of the analog hydrophone signal, an identification of a hydrophone that generated the acoustic information, and a time that the acoustic information is received from the hydrophone. Each data unit accumulator further includes an input interface that is operable during the first period for receiving an alert signal with a hydrophone analog signal, indicating that the associated source has data available. The input interface compares a current state of the alert signal to a previous state for detecting an occurrence of the assertion of the alert signal. The data unit accumulator also receives and stores a unit of data from a data source having an asserted alert signal, and is responsive to the storage of the unit of data therein, during the first period, to receive and store, during the second period, other information associated with the unit of data stored during the first period.
U.S. Pat. No. 5,450,549, issued Sep. 12, 1995, to Casparian, discloses a multi-port buffer that stores digitized image and/or audio information from a video camera and transfers the stored image information to a plurality of output channels. Digitized input data is passed through a crossbar switch and stored in a random access memory (RAM). The image data is retrieved from RAM and passed the crossbar switch to one of a plurality of first-in, first-out (FIFO) registers. Raster scan lines are passed from the FIFO registers to corresponding output channels. The order and rate of writing to RAM and reading out to the FIFO registers is controlled by an asynchronous queuing arbiter. If one of the output channels is slower than the others or operates at a variable clock speed, the asynchronous queuing arbiter changes the order in which the FIFO registers are filled to accommodate that output channel. Should one of the output channels fail, the bus request for the corresponding FIFO register is disabled, thereby skipping the failed channel.
U.S. Pat. No. 5,784,339, issued Jul. 21, 1998, to Woodsum et al, discloses a communication and position determining device for use in an underwater communication and position determining system wherein the device transmits a sequence of one or more data words, wherein each data word includes a plurality of data bits transmitted in parallel as a corresponding number of concurrently transmitted signals, each of which occupies a different frequency band. The signals have waveforms selected from a pair of fade resistant waveforms that can be discriminated from one another, to represent binary data and the frequency bands are separated from one another by guard bands having widths sufficient to prevent intersymbol interference between the signals representing the data bits of a data word due, for example, to doppler and phase/frequency shifting, and successive data words are separated sequentially in time by intervals having a duration sufficient to prevent intersymbol interference between the signals of successive data words due to multipath reverberation. A single one of the signals of a data word is used for position determination wherein two devices each determine the bearing to one another from the bearing of the signals received from the other and a round trip propagation time determined from the time delay in transmitting an interrogation data word, a first response data word and a second response data word.
U.S. Pat. No. 5,844,951, issued Dec. 1, 1998, to Proakis et al, discloses a method and apparatus for multichannel combining and equalization in a multichannel receiver. The receiver jointly performs diversity combining, equalization and synchronization. The method and apparatus may be used to provide a reduced complexity adaptive multichannel receiver for use in a digital communication system.
U.S. Pat. No. 6,289,284, issued Sep. 11, 2001, to Yamamoto, discloses a non-destructive method of measuring physical characteristics of a medium, such as uncemented sediment, sandstone, or limestone. A pseudo-random code is generated and is used to generate a pseudo-random acoustic signal. This signal is transmitted into the medium to be measured through the use of a transducer, such as a piezoelectric element, and is received by a plurality of hydrophones. The received signal is then processed to obtain an image of its velocity and attenuation. A universal geoacoustic model of the medium for a given set of measured data is determined, and the model is solved to obtain a pair of permeability-porosity results for the medium. The one of this pair of permeability-porosity results which is correctly indicative of the physical characteristics of the medium is then determined.
U.S. Pat. No. 7,110,678, issued Sep. 19, 2006, to Willebrand et al, discloses a hybrid wireless optical and radio frequency (RF) communication link that utilizes parallel free-space optical and RF paths for transmitting data and control and status information. The optical link provides the primary path for the data, and the RF link provides a concurrent or backup path for the network data, as well as a reliable and primary path for the control and status information. When atmospheric conditions degrade the optical link to the point at which optical data transmission fails, the hybrid communication link switches to the RF link to maintain availability of data communications. The switch may occur automatically, based on an assessment of the quality of the optical signal communicated through the optical path.
U.S. Pat. No. 7,177,232, issued Feb. 13, 2007, to Hagerty, discloses a wireless hydrophone system that includes a hydrophone joined to a preamplifier. A serial A/D converter receives the amplified hydrophone signal and provides a serial digital output representative of the signal. The A/D converter is joined to a processor which provides a start signal and a clock signal to the A/D converter. A digital transmitter is also controlled by the processor. The transmitter receives the serial digital output from the A/D converter for wireless transmission over an antenna. The system can also include logic for allowing the processor to provide an extended sync signal for transmission. The extended sync signal can alert a receiver to an initial transmission. The system can be incorporated in a hull treatment for positioning on a vessel's hull.
U.S. Pat. No. 7,362,653, issued Apr. 22, 2008, to Green et al, discloses a method and apparatus for determining the geophysical position of an autonomous underwater system utilizing underwater acoustic modems that exchange broadband underwater acoustic signals. The method includes the steps of initiating an exchange of broadband acoustic signals between the autonomous system of unknown geophysical position and a base system of known geophysical position wherein the depths of both systems is known. A bearing calculation is made on one of the signals transmitted between the systems, preferably through the use of an array of hydrophones placed closely together at predetermined locations on either the autonomous or base system. Also, the range between the two systems is determined by measuring the time of travel of at least one signal. By the acoustic transmission and sharing of information, as needed, about the known depths of the systems, the known geophysical position of the base system, and the range between the systems, sufficient data is gathered at one or both systems and used to determine the geophysical position of the autonomous system.
U.S. Pat. No. 7,529,304, issued May 5, 2009, to Hagerty, discloses a data transmission system that includes a serial A/D converter and a transmission processor. Transmission processor provides control signals to the A/D converter and first and second transmitters. The first transmitter is joined to the A/D converter to transmit a sync signal at a first frequency. The second transmitter is joined to transmit serial digitized data at a second frequency. First and second receivers are used to receive these frequencies. A reception processor is joined to the first receiver to activate a D/A converter on receipt of the sync signal. The D/A converter then converts digitized data received by the second receiver back to analog format. A method is also provided for transmitting and decoding the digital data.
U.S. Patent Application Publication No. 2005/0088916, published Apr. 28, 2005, to Zhu et al, discloses a method and a system of a high code speed low error probability underwater acoustic coherent communication for underwater transferring instruction, data and image. The communication system includes a host machine installed on a mother ship or a main control underwater vehicles A and a guest machine installed on an underwater vehicle B, wherein the host machine comprises an electronic subassembly, a transducer and a receiving line array which is vertically deployed and consists of more than two hydrophones, and the guest machine comprises an electronic subassembly and a transmitting/receiving transducer. The signal processing method is based on the joint technology of the space diversity, the self-optimized adaptive decision feedback equalizer and self-optimized adaptive phase tracker so as to overcome the affection of motion of the channel and the vehicles, such that the received signal could be quite close to the transmitted signal, and the bit error probability is low.
The above cited prior art does not disclose a software free system which is operable to quickly synchronize with and demultiplex frames of digital words. The solutions to the above described and/or related problems have been long sought without success. Consequently, those skilled in the art will appreciate the present invention that addresses the above and other problems.